1. Field of the Invention
The invention relates to fans for cooling engines and to the control of such fans for accurately matching the cooling needs of the engine.
2. Description of the Related Art
Engines, such as the engines of automotive vehicles, generate heat during use and are subject to broad ranges of environmental heat. Accordingly, engines must be cooled to prevent overheating. The cooling needs of the engine are dependent upon characteristics of the engine, ambient temperature and the operating speed of the engine. Most engines are cooled by a liquid coolant that flows through channels in proximity to the engine. Heat from the engine is transferred to the coolant, and the heated coolant then passes through a heat exchanger or radiator. A fan directs cool air through the radiator to effect the cooling of the liquid.
The cooling rate is dependent partly upon the temperature of the cooling liquid, the temperature of the air and the rate of flow of the cooling air. The rate of flow of cooling air is dependent upon the rotational speed of the fan, and the speed of the vehicle.
Some prior art engines are configured to have the fan rotate sufficiently fast to accommodate all anticipated cooling needs. However, the fan is driven by the engine, and hence the engine operates less efficiently when there is more energy diverted to the operation of the fan. Additionally the rotating fan creates noise that is roughly proportional to the rotating speed of the fan. Thus, the fans in these prior art systems often generate more fan noise than is required.
Problems associated with fan noise and engine operating efficiencies are well known in the art, and there have been many efforts to match the fan speed to the cooling needs of the engine. The assignee of the subject invention has done considerable work in the field of fan fluid clutches that have proved to be very effective in matching the fan speed to the cooling needs of the engine.
The typical fan fluid clutch has a driving disc fixedly mounted to a rotating shaft of an engine. Thus, the driving disc rotates at the speed of the shaft of the engine. A housing is mounted rotatably to the shaft by bearings. Thus, the rotating shaft does not rotate the housing directly. Fan blades are mounted to the exterior of the housing. The interior of the housing includes a torque transmission chamber that surrounds the drive disc and an oil reservoir spaced from the torque transmission chamber. However, one or more oil supply holes and an oil recirculation passage provide communication between the oil reservoir and the torque transmission chamber.
A torque transmission fluid is placed in the oil reservoir and can flow into the torque transmission chamber via the oil supply hole between the oil reservoir and the torque transmission chamber. The torque transmission fluid transmits torque from the drive disc to the housing in proportion to the contact area of the torque transmission fluid with both the drive disc and the housing.
A valve is mounted in the housing and controls the oil supply hole between the oil reservoir and the torque transmission chamber. The valve opens when cooling needs are high. Hence, more oil will flow into the torque transmission chamber at these times. The greater amount of oil in the torque transmission chamber results in greater torque transmission between the drive disc and the housing. Under these conditions, the housing and the fan blades thereon rotate faster to provide more cooling. The valve closes the oil supply hole between the oil reservoir and the torque transmission chamber when cooling needs are low. As a result, less oil will flow into the torque translation chamber and the drive disc will transmit less torque to the housing. Under these conditions, the housing rotates more slowly and the fan blades on the housing direct less cooling air towards the engine. Additionally less energy is diverted from the engine to the fan when cooling needs are low, and therefore the engine operates more efficiently.
Fan fluid clutches are described extensively in the patent literature, including many U.S. patents assigned to the assignee of the subject invention. The simplest of these devices provides a temperature sensor, such as a bi-metal strip, at an exterior position on the housing. The temperature sensor communicates with the valve to open the oil supply hole during periods of high temperature and to close the oil supply hole during periods of lower temperatures.
More sophisticated fan fluid clutches have been developed in recent years and provide more inputs for controlling the movement of the valve that opens and closes the oil supply hole between the oil reservoir and the torque transmission chamber. These more sophisticated systems measure conditions such as: ambient temperature, vehicle speed, engine speed, air conditioning operating parameters, transmission oil temperature and the like. A controller is programmed with logic to determine a target fan speed (TFS) based on an analysis of these inputs. The controller then operates an electromagnet to move the valve relative to the oil supply hole. More particularly, the controller is programmed and calibrated to turn the electromagnet on and off for opening and closing the valve at a “duty rate” (DR) that is intended to achieve the appropriate volume of oil in the torque transmission chamber, and hence to achieve the target fan speed TFS. A prior art system that provides for outside control of the fan coupling is shown, for example in U.S. Pat. No. 6,550,596 and U.S. Pat. No. 6,915,888, which are assigned to the assignee of the subject invention. The disclosures of U.S. Pat. No. 6,550,596 and U.S. Pat. No. 6,915,888 are incorporated herein by reference.
Some recent efforts in the field of externally controlled fan drives employ “smart fluid” in a torque transmission chamber. These systems use a constant volume of fluid in the torque transmission chamber, and hence avoid the use of condition responsive valves to control the volume of fluid in the torque transmission chamber. Rather, these recent efforts attempt to control the torque transmission characteristics of a fixed volume of fluid. For example, a magnetic field can be applied in a controlled manner to alter the viscosity of the torque transmission fluid. Examples of these externally controlled fan drives are described in U.S. Pat. No. 5,960,918, U.S. Pat. No. 6,032,772, U.S. Pat. No. 6,102,177, U.S. Pat. No. 6,318,531 and U.S. Pat. No. 6,585,092, the disclosures of which are incorporated herein by references.
Prior art externally controlled fan devices (ECFD's) are better at achieving the target fan speed when the engine and the fan coupling apparatus are new. However, the ability to achieve the target fan speed TFS deteriorates over time. This deterioration is due to gradual wear of components in the fan coupling device and/or changes in the viscosity of the torque transmission fluid. These externally controlled fan devices measure the actual fan speed AFS and compare the actual fan speed AFS to the target fan speed TFS. These devices then change the duty rate DR in an effort to narrow or eliminate the difference between the target fan speed TFS and the actual fan speed AFS. However, the control logic in the prior art controller is calibrated based on new fan coupling apparatus and hence may not consistently match the target fan speed TFS if the externally controlled fan drive undergoes deterioration over time. As a result, these recent prior art externally controlled fan drives tend to swing the actual fan speed AFS substantial amounts to one side or the other of the target fan speed TFS. Prior art externally controlled fan drives that focus directly on the duty rate DR often provide less cooling than is required or more cooling, and hence less efficient engine operation. U.S. Pat. No. 6,807,926 is one example of a prior art externally controlled fan drive that includes actual fan speed as an input to the controller. This system has plural oil supply holes and plural valves. The duty rate DR of the respective valves are changed independently of one another in an effort to bring the actual fan speed AFS closer to the target fan speed TFS. However, a plural valve system is more complex then a single valve system. Furthermore, externally controlled fan drives that focus primarily on the duty rate DR are known to swing widely on one side or the other of the target fan speed TFS, as explained above.
Accordingly, an object of the subject invention is to provide an outside control-type fan coupling apparatus that provides optimum cooling for an engine substantially uniformly over a long period of time.